Field of the Invention
This invention relates, in general, to calibrated gases and their uses and, in particular, to a new and useful calibrating gas generator and to a method of forming a calibrating gas.
Breath-alcohol meters are used increasingly by the police in their patrols. In order to ensure the measuring accuracy, particularly in view of later court proceedings, the breath-alcohol meters are regularly calibrated. To this end is used a calibrator which is of high concentration stability, simple and inexpensive to operate.
A known calibrating gas generator contains two tanks which are filled with an aqueous ethyl alcohol standard. They are thermostated to the same temperature of 34.+-.0.2 deg C. The tanks are series-connected over lines with a pump. The pump conveys ambient air through the filling of the first and subsequently of the second tank. The air absorbs alcohol and water vapor from the liquid, the calibrating gas formed has a defined alcohol concentration which depends on the temperature of the tank. The concentration of the liquid drops, due to the alcohol given off. This effects particularly the first tank. From the second tank is only taken the amount of alcohol which the liquid in the first tank could not absorb, due to the impoverishment of the liquid which had already occurred there. This results in a gradual drop of the liquid concentration in the second tank too, which leads to differences in the concentration of the calibrating gas. Within the framework of the required tolerances, the amount of the calibrating gas that can be produced with a liquid filling must therefore be limited. The frequent replacement of the partly utilized liquid is annoying for the operator, due to the energy expenditure and the waiting periods until it can be operated again, because of the required heating (Operating Instructions 4752.01, Oct. 1981, Draegerwerk AG).